The White House says it now has definitive proof Syrian president Bashar al-Assad has used chemical weapons in his conflict with rebel fighters.

US president Barack Obama's deputy national security advisor, Ben Rhodes, says US intelligence has confirmed Mr Assad's regime has deployed the nerve agent Sarin on a small scale against the Syrian opposition on multiple occasions in the past year.

He says US intelligence officials estimate between 100 and 150 people have died from chemical weapons attacks in Syria to date.

He added that the US has no "reliable" evidence that rebels had used such weapons.

Key points

The US says the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale multiple times

US intelligence estimates up to 150 people have died from detected chemical weapons attacks

Washington says the attacks cross its "red line" in the crisis

President Barack Obama has decided to further support rebel forces, including militarily

United Nations says 93,000 people - including 6,500 children - have died in the Syrian conflict

"Our intelligence community assesses that the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year," Mr Rhodes said in a White House statement.

"The president has been clear that the use of chemical weapons - or the transfer of chemical weapons to terrorist groups - is a red line for the United States.

"The president has said that the use of chemical weapons would change his calculus, and it has."

In a conference call with reporters, Mr Rhodes said Washington had shared its information with Russia about the use of chemical arms, but that Moscow had not yet agreed Mr Assad should step down.

Mr Rhodes did not say if the US was moving towards directly arming the rebels battling Mr Assad's regime, but said Mr Obama "will be consulting with Congress on these matters in the coming weeks".

"The United States and the international community have a number of other legal, financial, diplomatic, and military responses available," he said.

"We are prepared for all contingencies, and we will make decisions on our own timeline. We're going to act very deliberately."

Prime Minister Julia Gillard says Australia's security personnel also believe there is evidence that chemical weapons have been used in Syria.

"As the White House has said today there is evidence that chemical weapons have been used in Syria and that is the view of our own security personnel as well," she said.

"So we would all condemn the use of chemical weapons and we would all be calling for the regime in Syria to let the UN inspection team in."

Defence Minister Stephen Smith said the Assad regime needed to stop using sarin immediately, but that there would not be an immediate move to arm rebels.

"It is clear that what should now occur is that the Assad regime should undertake to desist from use or contemplation of use of chemical weapons. They should allow the United Nations investigation team to go in to do the investigation," he told ABC News 24.

"The statement made by the US essentially says that they are now looking at the array of diplomatic and military options. But they've also made it clear they're taking it step-by-step."

Mr Rhodes said both the use of chemical weapons, and the increased involvement of Hezbollah and Iran in the conflict, had "added an element of urgency" to the process.

"The president has made a decision about providing more support to the opposition," he said.

"That will involve providing direct support to the (rebel) Supreme Military Council. That includes military support."

'More decisive action needed'

Senior Republican senators John McCain and Lindsey Graham welcomed the administration's move on the issue, but said in a statement that "more decisive actions" were now needed to end the spiralling violence in Syria.

"It's been reported to me by reliable sources that the president ... has decided, he's reached the conclusion that they used chemical weapons, and that they are going to provide arms to the rebels," Mr McCain said.

"Since he hadn't announced it, he may reverse that. But I've been told that that's the case."

Earlier, a US defence official said the US would keep F-16 fighter jets and Patriot anti-missile weapons in Jordan - which borders Syria - after a joint military exercise ends this month.